1. Field of the Invention
The present invention relates to a multiple-wavelength light source for use as a light source for a wavelength-division-multiplexing (WDM) optical communication system, and a method of controlling oscillation frequencies of such a multiple-wavelength light source.
2. Description of the Related Art
Heretofore, one conventional multiple-wavelength light source for use as a light source for a WDM optical communication system comprises a multiple-wavelength semiconductor laser as shown in FIG. 1 of the accompanying drawings.
The conventional multiple-wavelength semiconductor laser, which has been reported by Tanaka et al. in collected preprints for Electronic Communications Society General Conference, 1997, C-3-160, has four spot size convertor integrated lasers (hereinafter referred to as "SS-LD"), four optical waveguides (PLC: Planar Lightwave Circuits) of silica, and diffraction gratings produced by UV photolithography, all integrated on an Si substrate. It has been confirmed that the conventional multiple-wavelength semiconductor laser performs simultaneous oscillation at four wavelengths in a single mode.
More specifically, as shown in FIG. 1, the conventional multiple-wavelength semiconductor laser comprises four SS-LDs 102 for emitting laser beams having respective oscillation wavelengths, four optical waveguides 103 for leading the laser beams emitted by SS-LDs 102 to output end face 105, and four diffraction gratings 104 disposed in the respective optical waveguides 103 and serving as external cavities for SS-LDs 102. SS-LDs 102, optical waveguides 103, and diffraction gratings 104 are integrated on Si substrate 101. Output end face 105 is coated with an antireflection coating for minimizing reflection of the laser beams. While the conventional multiple-wavelength semiconductor laser shown in FIG. 1 emits laser beams having four oscillation wavelengths .lambda.1-.lambda.4, it can produce more oscillation wavelengths by adding one or more elemental structures in a parallel arrangement.
Each of SS-LDs 102 has a tapered waveguide for narrowing the radiation angle of the emitted laser beam thereby to reduce an optical coupling loss with optical waveguide 103. Use of SS-LDs 102 is effective to lower the cost of the multiple-wavelength semiconductor laser because it does not require optical lenses for changing the radiation angles of the emitted laser beams.
Each of diffraction gratings 104 is fabricated by exposing an SiO.sub.2 layer (which will become an upper cladding layer) above the core of optical waveguide 103 to interference UV radiation. Structural details, including a grating pitch and a shape, of diffraction gratings 104 determine the oscillation wavelengths of the laser beams output from output end face 105.
However, the conventional multiple-wavelength semiconductor laser has been problematic in that since the oscillation wavelengths of the multiple-wavelength semiconductor laser are determined by the fabrication accuracy of the diffraction gratings, the yield of fabricated devices is poor if they are to be used as a light source for a WDM optical communication system because such a light source needs accurate wavelength settings.